This invention generally relates to the field of sample preparation for cross-section analysis. More particularly, the present invention pertains to an improved method of preparing a metallographic sample for inspection, by grinding a surface of the sample.
The following disclosure describes how the present invention applies to the field of semiconductor devices. However, the invention is not limited to semiconductor device applications, but applies to any sample to be cross-sectioned where an externally observable feature identifies the desired location of the cross-section. Other applications include, but are not limited to metals, ceramics, glass, plastics, composites, etc.
Semiconductor devices comprise a plurality of features formed on a semiconductor wafer. Semiconductor devices typically comprise a plurality of layers made up of conductive and insulative patterns, vias, and trenches. In order to function properly, the semiconductor device layers must be accurately aligned with each other, and sound electrical contacts must be formed with the conductive patterns. Inspections are performed on semiconductor devices as both part of routine quality control, and when trouble shooting to determine the cause of a semiconductor device failure. Because semiconductor devices comprise a plurality of layers, features internal to the semiconductor device are not readily observable by visual inspection. In order to inspect internal features, a cross-section of the semiconductor device is viewed.
Internal features that are inspected include flip chip/package solder bonds, feature and layer thicknesses, microstructure characterization, and the alignment of conductive layers and interconnects and contacts. Possible failure mechanisms that need to be inspected include the presence of voids in welds and solder bonds; layer separation, i.e., delamination or debonding of layers; and misregistration of device features.
The term semiconductor devices as used herein is not be limited to the specifically disclosed embodiments. Semiconductor devices as used herein include a wide variety of electronic devices including flip chips, flip chip/package assemblies, transistors, capacitors, microprocessors, random access memories, etc. In general, semiconductor devices refer to any electrical device comprising semiconductors.
Typically, to inspect the interior of a semiconductor device, a section of the semiconductor device, containing an area of interest that is to be inspected, is cut from the semiconductor device. The section cut from the semiconductor device can be cut using a metallographic saw, such as a wire saw, diamond impregnated blades, silicon carbide blades, or other abrasive saws. A margin of semiconductor device surrounding the area of interest is left after cutting so that the cutting does not damage the area of interest that is to be inspected. The section of the semiconductor device containing the area of interest is often mounted on a suitable holder, such as a stub, which is supported by a chuck. Then the margin surrounding the area of interest is removed by grinding. A grinding wheel or belt with a suitable grinding media is used to grind the sample. As the margin is ground away and the grinding wheel approaches the area of interest, the grinding media is successively changed to a finer grit material. In the final stages of grinding, polishing of the sample is performed. As used in the instant specification and claims the term xe2x80x9cgrindingxe2x80x9d includes polishing.
The section of the semiconductor device being inspected may be mounted on a metallic or plastic stub with either two-sided tape or an adhesive, such as a thermal adhesive. The stub is mounted in a chuck, which supports the sample while it is undergoing grinding.
A sample can also be encased or potted within a transparent polymer resin. When potted, the sample can be held manually or clamped in a sample fixture when grinding.
In the prior art method of grinding semiconductor device samples, the grinding has to be stopped frequently, the sample removed from the chuck, and visually inspected with a microscope to determine whether the area of interest has been reached. The danger exists that grinding can proceed too far and either damage or grind right through the area of interest. The prior art process is inefficient and time consuming because the grinding process has to be interrupted each time the sample is inspected to determine whether grinding is complete.
There exists a heed in the metallographic sample inspection art to eliminate the problem of over-grinding a sample. There exists a need in this art to perform real-time monitoring of the grinding process to determine how fast grinding is progressing and to determine when the area of interest is reached. There further exists a need in this art to determine when to change the grit media to finer media without having to remove the sample from the chuck and perform a visual inspection of the sample.
These and other needs are met by the embodiments of the present invention, which provide an arrangement for grinding a metallographic sample comprising a metallographic sample, containing an area of interest, with first and second opposing major sides. An imaging arrangement is positioned so as to generate images of the first major side of the semiconductor device sample while the sample is undergoing grinding. A grinding wheel is provided for grinding a surface of the sample.
The earlier stated needs are also met by another embodiment of the instant invention which provides a method of real-time monitoring of the grinding of a metallographic sample comprising: providing a metallographic sample, containing an area of interest, with first and second opposing major sides. The sample is positioned so that a surface approximately normal to the opposing major sides can be ground. An imaging arrangement is positioned to image the first side of the sample while the sample is being ground. A side of the sample approximately normal to the opposing major sides undergoes grinding to approach the area of interest in the semiconductor device. The first side of the sample is imaged while the sample is undergoing grinding to monitor grinding progress.
The earlier stated needs are further met by another embodiment of the instant invention which provides an apparatus for monitoring the grinding of a metallographic sample comprising an imaging arrangement mounted on one surface of a substantially transparent substrate. The imaging arrangement comprises a lens and video camera located along a common optical path with the substantially transparent substrate.
The present invention provides real-time monitoring of the grinding of a metallographic sample by imaging the sample being ground. The imaging arrangement includes a video camera for imaging the sample. In certain embodiments, the substantially transparent substrate provides support for both the imaging arrangement and the sample, and allows the video camera to be positioned away from the grinding area. To prevent damage to the camera, fiber optic tapers, and fiber optic lines comprising fiber optic tubes or cables can be used to further remove the video camera from the grinding area.
To improve image resolution, an artificial light source is used in certain embodiments to illuminate the sample being ground. The light source can either be located in the optical path of the imaging arrangement or it can be a remote light source. The video camera records the image and can either send the output to a video monitor for real-time display or transmit the data to a computer, which captures the image and stores the image data.
Some advantages of the instant invention include the ability to perform real-time monitoring of the grinding process. The sample being ground does not need to be removed and visually inspected to determine how far grinding has progressed. The apparatus and method of the present invention provide a more efficient inspection process. Use of real-time monitoring prevents over-grinding of the sample and the resulting loss of the area of interest. Inspection of metallographic samples is a labor intensive, time-consuming process. The present invention is more efficient because the grinding process is not interrupted to check the sample to see how far grinding has progressed and sample loss because of over-grinding is eliminated.
The foregoing and other features, aspects, and advantages of the present invention will become apparent from the following detailed description of the present invention when taken in conjunction with accompanying drawings.